In the related art, image forming apparatuses using a system for performing printing on one side of a sheet and then performing printing on the other side while printing on one side of another sheet, i.e. a system for consecutively performing single-sided printing on a plurality of sheets (hereinafter, referred to as a system of conveying a plurality of sheets at once), as a sheet conveying system for performing double-sided printing, are known.
Image forming apparatuses each of which supports a plurality of conveying systems are known. For example, there has been disclosed a technology for performing switching between conveying systems on the basis of sheet jam occurrence probabilities in an image forming apparatus supporting a plurality of conveying systems different in the maximum number of sheets which can exist in a conveying mechanism at the same time.
It is susceptible for an electrophotographic image forming apparatuses to be improved in control on sheet conveyance for the double-sided printing. The difference between the current temperature of a rotator of a fixing unit and a fixing temperature is small just after the temperature of the rotator reaches the fixing temperature. A temperature at which it is possible to adequately fix a toner image on one sheet is set as the fixing temperature. Therefore, in a system of conveying a large number of sheets per unit time like the above-mentioned system of conveying a plurality of sheets at once, when the rotator loses heat due to passage of a sheet, whereby the temperature of the rotator becomes lower than the fixing temperature, the rotator may not adequately recover the temperature required to secure fixing strength until the next sheet arrives. On the other, although it is easy for a system conveying a small number of sheets per unit time to secure a time required to recover the temperature of a rotator, productivity is low.